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      <title>JVM 如何处理并发GC与应用线程的协调问题？ - 交互式测验</title>
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              <h1>JVM 如何处理并发GC与应用线程的协调问题？ - 交互式测验</h1>
              <p>检验你对文档内容的理解程度</p>
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  {
    "question": "在JVM的垃圾回收机制中，“Stop-the-World” (STW) 事件具体指的是什么？",
    "options": [
      "只有执行GC的线程被暂停，应用线程继续运行。",
      "JVM因为严重错误而完全停止，需要手动重启。",
      "在垃圾回收期间，所有的应用线程都被暂停，直到GC任务完成。",
      "只有创建新对象的线程被暂停，已有线程不受影响。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "“Stop-the-World”是JVM垃圾回收过程中的一个关键概念，它指的是为了保证垃圾回收能够在一个一致的内存快照上进行，GC线程会暂停所有正在运行的应用线程。虽然这会影响应用的响应性，但对于串行和并行回收器来说是必需的步骤。像CMS和G1这样的并发回收器则致力于通过并发处理来缩短这个暂停时间。",
    "source": "文档中「2.1 串行GC（Serial GC）」部分",
    "hint": "文档中描述串行GC的协调方式时提到了一个关键术语，它解释了当GC发生时应用线程的状态。"
  },
  {
    "question": "哪种垃圾回收器（GC）的主要目标是通过与应用线程并发执行来最小化应用暂停时间，但可能会因其回收算法而产生内存碎片问题？",
    "options": [
      "串行GC (Serial GC)",
      "并行GC (Parallel GC)",
      "G1 GC (Garbage-First GC)",
      "CMS GC (Concurrent Mark-Sweep GC)"
    ],
    "correctAnswerIndex": 3,
    "explanation": "CMS (Concurrent Mark-Sweep) GC的设计目标就是为了获得最短的回收停顿时间。它在“并发标记”和“并发清除”阶段允许应用线程继续运行。然而，由于其清除阶段是基于标记-清除算法，不会对存活对象进行移动和整理，因此长时间运行后可能会在老年代产生内存碎片，影响大对象的分配。",
    "source": "文档中「2.3 并发标记清除GC（Concurrent Mark-Sweep GC, CMS）」部分",
    "hint": "寻找一种以“并发”为主要特点，但也提到了“碎片问题”作为其潜在缺点的垃圾回收器。"
  },
  {
    "question": "与其他垃圾回收器相比，G1 GC (Garbage-First GC) 的一个显著优势和协调方式是什么？",
    "options": [
      "它使用单线程进行回收，实现简单，适用于小型应用。",
      "它总是会暂停所有应用线程，并使用所有CPU核心进行回收，以追求最大吞吐量。",
      "它将堆内存划分为多个区域，并设定一个“停顿时间目标”，通过增量和并发的方式进行回收。",
      "它完全消除了“Stop-the-World”，GC过程对应用线程完全透明。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "G1 GC的核心设计思想是平衡停顿时间和吞吐量。它将Java堆划分为多个大小相等的独立区域（Region），并跟踪这些区域里垃圾的价值大小。在回收时，它会优先回收价值最大的区域（这也是“Garbage-First”的由来）。最重要的是，用户可以为其设定一个期望的停顿时间目标，G1会尽力在这个目标内完成回收，这使得GC停顿变得更加可预测。",
    "source": "文档中「2.4 G1 GC（Garbage-First GC）」部分",
    "hint": "G1 GC的一个关键特性是它不针对整个堆进行回收，并且允许用户设定一个关于性能的目标。"
  },
  {
    "question": "在并发GC（如CMS或G1）执行期间，应用线程可能仍然在修改对象的引用。JVM使用哪种机制来确保GC能够正确地跟踪这些变化，防止误删活对象？",
    "options": [
      "逃逸分析 (Escape Analysis)",
      "写屏障 (Write Barriers)",
      "停顿时间控制 (Pause Time Control)"
    ],
    "correctAnswerIndex": 1,
    "explanation": "写屏障是一种关键的协调机制。当应用线程修改对象引用（例如 `obj.field = new_obj`）时，写屏障会拦截这个写操作，并执行一些额外的逻辑，比如记录下这次引用关系的变化。这样，并发执行的GC线程就能知道哪些对象的状态发生了改变，从而保证了垃圾回收的准确性。",
    "source": "文档中「3. 协调机制」部分",
    "hint": "文档的“协调机制”一节中，描述了一种用于“记录对象的引用变化”的技术。"
  },
  {
    "question": "串行GC (Serial GC) 和并行GC (Parallel GC) 的共同点是什么？",
    "options": [
      "它们都主要用于大堆内存和需要低延迟的场景。",
      "它们在执行垃圾回收时，都会触发“Stop-the-World”事件，暂停所有应用线程。",
      "它们都可以在垃圾回收的大部分阶段与应用线程并发执行。",
      "它们都使用“写屏障”技术来跟踪对象引用。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "尽管并行GC使用多线程来加速回收过程，从而缩短了停顿时间，但它和串行GC一样，其回收阶段都需要完全暂停应用线程。这个暂停事件就是“Stop-the-World”。并发执行是CMS和G1等更现代的回收器的特性。",
    "source": "文档中「2.1 串行GC（Serial GC）」和「2.2 并行GC（Parallel GC）」部分",
    "hint": "比较这两种GC的“协调方式”，它们都提到了一个共同的、会导致应用暂停的事件。"
  },
  {
    "question": "文档中提到的“逃逸分析”是一种JVM高级优化技术，它的主要作用是什么？",
    "options": [
      "分析并预测下一次GC的停顿时间，以实现自适应调节。",
      "分析对象的引用链，以防止潜在的内存泄漏。",
      "识别生命周期仅限于方法内部的对象，并将其在栈上分配以减轻GC压力。",
      "在并发标记阶段，分析哪些对象可能从“非存活”变为“存活”。"
    ],
    "correctAnswerIndex": 2,
    "explanation": "逃逸分析是编译器层面的一项重要优化。如果JVM通过分析发现一个对象的作用域永远不会“逃逸”出创建它的方法（即不会被返回，也不会被赋值给外部引用），那么这个对象就可以被认为和方法的生命周期绑定。此时，JVM可以不把它分配在需要GC管理的堆上，而是直接在当前线程的栈上分配。栈上分配的内存在方法结束时会自动被回收，因此极大地降低了GC的频率和压力。",
    "source": "文档中「4. 高级特性」部分",
    "hint": "这项技术关注的是对象的分配位置，旨在“减少堆内存分配”。"
  }
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        }

        function reviewQuiz() {
            const reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }

            document.querySelector('.quiz-body').style.display = 'block';
            document.getElementById('quiz-footer').style.display = 'flex';
            
            currentQuestionIndex = 0;
            loadQuestion();
            updateNavigationButtons(); // Ensure nav buttons are correctly displayed
        }

        // Event Delegation for options
        optionsListEl.addEventListener('click', (event) => {
            if (event.target && event.target.matches('li.option-item')) {
                // Check if the question has already been answered
                if (userAnswers[currentQuestionIndex] === null) {
                    const selectedIndex = parseInt(event.target.dataset.index, 10);
                    selectOption(selectedIndex);
                }
            }
        });

        prevBtn.addEventListener('click', goToPreviousQuestion);
        nextBtn.addEventListener('click', goToNextQuestion);
        hintBtn.addEventListener('click', showHint);
        
        // 初始加载
        loadQuestion();
      </script>
    </body>
    </html>
  